This invention relates generally to magnetic recording heads and more particularly to a structure for a ferrite driven, surface thin-film magnetic recording head wherein a substantial portion of the ferrite core has been replaced with a magnetically impermeable material and an optimal back-bar arrangement which reduces the inductance and increases the efficiency of the head.
while variety of data storage mediums are available, magnetic tape remains a preferred forum for economically storing large amounts of data. In order to facilitate the efficient use of this media, magnetic tape will have a plurality of data tracks extending in a transducing direction of the tape. Once data is recorded onto the tape, one or more data read heads will read the data from those tracks as the tape advances, in the transducing direction, over the read head. It is generally not feasible to provide a separate read head for each data track, therefore, the read head(s) must move across the width of the tape (in a translating direction), and center themselves over individual data tracks. This translational movement must occur rapidly and accurately.
In order to facilitate the controlled movement of a read head across the width of the media, a servo control system is generally implemented. The servo control system consists of a dedicated servo track embedded in the magnetic media and a corresponding servo read head (which is usually one of the standard read heads, temporarily tasked to servo functions) which correlates the movement of the data read heads.
The servo track contains data, which when read by the servo read head is indicative of the relative position of the servo read head with respect to the magnetic media in a translating direction. In one type of traditional amplitude based servo arrangement, the servo track was divided in half. Data was recorded in each half track, at different frequencies. The servo read head was approximately as wide as the width of a single half track. Therefore, the servo read head could determine its relative position by moving in a translating direction across the two half tracks. The relative strength of a particular frequency of servo signal would indicate how much of the servo read head was located within that particular half track. The trend toward thinner and thinner magnetic tape layers causes amplitude modulation problems with this and other amplitude based heads. That is, as the thickness of the magnetic layer decreases, normal variations on the surface represent a much larger percentage variation in the magnetic layer, which will dramatically affect the output signal.
Recently, a new type of servo control system was created which allows for a more reliable positional determination by reducing the amplitude based servo signal error traditionally generated by debris accumulation, media thickness non-uniformity and head wear. U.S. Pat. No. 5,689,384, issued to Albrecht et al. on Nov. 19, 1997, introduces the concept of a timing based servo pattern on a magnetic recording head.
In a timing based servo pattern, magnetic marks (transitions) are recorded in pairs within the-servo track. Each mark of the pair will be angularly offset from the other. For example, a diamond pattern has been suggested and employed with great success. The diamond will extend across the servo track in the translating direction. As the tape advances, the servo read head will detect a signal or pulse generated by the first edge of the first mark. Then, as the head passes over the second edge of the first mark, a signal of opposite polarity will be generated. Now, as the tape progresses no signal is generated until the first edge of the second mark is reached. Once again, as the head passes the second edge of the second mark, a pulse of opposite polarity will be generated. This pattern is repeated indefinitely along the length of the servo track. Therefore, after the head has passed the second edge of the second mark, it will eventually arrive at another pair of marks. At this point, the time it took to move from the first mark to the second mark is recorded. Additionally, the time it took to move from the first mark (of the first pair) to the first mark of the second pair is similarly recorded.
By comparing these two time components, a ratio is determined. This ratio will be indicative of the position of the read head within the servo track, in the translating direction. As the read head moves in the translating direction, this ratio will vary continuously because of the angular offset of the marks. It should be noted that the servo read head is relatively small compared to the width of the servo track. Ideally, the servo head will-also be smaller than one half the width of a written data track. Because position is determined by analyzing a ratio of two time/distance measurements, taken relatively close together, the system is able to provide accurate positional data, independent of the speed (or variance in speed) of the media.
Of course, once the position of the servo read head is accurately determined, the position of the various data read heads can be controlled and adjusted with a similar degree of accuracy on the same substrate. Namely, the various read heads are fabricated on the same substrate with a known spacing between them. Hence knowing the location of one allows for a determination of the location of the remainder of the read heads.
When producing magnetic tape (or any other magnetic media) the servo track is generally written by the manufacturer. This results in a more consistent and continuous servo track, over time. To write the timing based servo track described above, a magnetic recording head bearing the particular angular pattern as its gap structure, must be utilized. As it is advantageous to minimize the amount of tape that is dedicated to servo tracks, to allow for increased data storage, and it is necessary to write a very accurate pattern, a very small and very precise servo recording head element must be fabricated.
Two types of servo recording heads having a timing based pattern are known. The first is a pure thin film head, such as that disclosed by Aboaf et al. in U.S. Pat. No. 5,572,392, issued on Nov. 5, 1996. With a pure thin film head, all of the components of the head are created from layering different materials, as thin films, on an inert substrate. For example, the magnetic core, the windings and any barrier materials are formed by producing thin films. Such a head has very low inductance, however, it is extremely difficult to manufacture. To date, pure thin film heads are generally not utilized for time based servo heads and are not seen as a practical way to produce such a magnetic head.
A very different type of recording head is taught by Albrecht et al. in the ""384 patent. This second type of head is referred to herein as a surface film or surface thin film head and is illustrated as 100, in FIG. 18. The surface film head 100 includes two C-shaped ferrite blocks 110, 112 that are bonded to a ceramic member 114 that extends the entire width of the head 100. A surface is then polished flat or contoured and prepared for this film deposition. A magnetically permeable thin film 116 is deposited over an upper surface of the ferrite blocks 110, 112 and the exposed upper portion of the ceramic member 114. The thin film I 16 is shown much larger than it would actually be, respective to the other elements. Gaps 118 are formed in the thin film 116, in an appropriate timing based pattern. Windings 120 are wrapped and are electrically driven to drive flux around the ferrite core and through the thin film 116 (in the direction of arrow A). The flux leaks from the gaps 118 and writes media passing over it.
Such a surface film head has a high inductance due to the large volume of ferrite forming the core and a high reluctance xe2x80x9cback-gapxe2x80x9d, due to the separation of the ferrite block 110, 112 by the ceramic member 114, on the underside of the head (i.e., opposite the thin film 116). The size and dimensions of the head are determined by the end use characteristics. For example, the width of the head 100 is defined by the width of the media; i.e., a head that is 19 mm wide is appropriate to support a tape that is 12.5 mm wide. The ceramic member 114 must be thick enough to allow the proper patterns 118 to be located above it and is approximately 0.012xe2x80x3 in the known versions of the Albrecht et al head, produced by IBM. The length of the head must be sufficient to support the media as it travels over the tape bearing surface and the depth (especially of the ferrite cores) must be sufficient to allow appropriate windings to be attached and to allow the head to be securely fixed in a head mount.
With the surface film head, flux is forced to travel through a magnetically permeable thin film that bridges a generally magnetically impermeable barrier between sections of the core. The writing gap is located within this thin film and the magnetic flux is expected to leak from this gap and write the media. The width of the ferrite is much larger than the sum of the channel widths. Hence, there is a large amount of unnecessary ferrite inductance. In other words, as a result of the relatively large amount of extraneous ferrite, an unnecessarily high amount of inductance is created. Therefore, to produce a relatively small amount of flux leakage through a small gap in the thin film, very high levels of current are required to generate sufficient magnetic flux throughout the relatively large core. This requires greater write current from the drive circuitry, lowers the frequency response of the head, and increases the rise time of the writing pulses from the head.
The thin film layer bridges a xe2x80x9cgapxe2x80x9d between the ferrite sections of the substrate, at one end of the head. This is, of course, the writing end of the head. This gap, referred to as the xe2x80x9csub-gapxe2x80x9d by the present inventor to distinguish it from the writing gaps in the thin film, is formed from and defined by the ceramic insert. As discussed above, the ceramic insert extends through the entire height of the write head. This forms a very large xe2x80x9cback-gapxe2x80x9d in a portion of the head opposite the thin film layer. This back-gap, in some prior recording heads, is a portion of the head where the magnetic flux must travel through the ceramic member in order to complete the magnetic circuit. Ultimately, this forms a barrier which hampers the magnetic flux; in other words the reluctance through the back-gap is relatively high and again, the head must be driven harder to compensate. This is only really problematic in heads having a larger back-gap with respect to the writing gap, such as in Albrecht et al. Various other magnetic heads, video for example, will have a back-gap length equal to the writing gap length. In addition, the video core back-gap is made very tall, thus increasing its area and reducing the back-reluctance. Techniques used to reduce the reluctance of video recording heads are not applicable to sub-gap driven heads.
Such problems are intensified with heads having multiple writing gaps, or channels. As shown in FIG. 18, a single core is driven and simultaneously writes multiple channels (each of the writing gaps 118 forms a separate channel). In order to do so, a multi-channel head will necessarily have to be wider than a single channel head. This in turn necessitates that the core become larger. All of this leads to a head having a larger amount of magnetically permeable material through which a predetermined amount of magnetic flux must flow while attempting to consistently and simultaneously write multiple channels. In other words, excess and unused ferrite material is provided in between the channels which unnecessarily increases the overall inductance of the head.
Therefore, there exists a need to provide an efficient multi-channel timing based head having a lower overall inductance, a lower reluctance through the back-gap, a higher frequency response, and greater efficiency. Furthermore, there exists a need to provide such a multi-channel head with the ability to individually and,separately drive and control each channel.
The present invention relates to a low inductance, high efficiency sub-gap, surface thin film magnetic recording head and a method of fabricating the same.
A substrate consisting of a ceramic member, glass bonded between a pair of ferrite blocks is prepared. After the substrate is created, it is diced to form a base from which a plurality of columns extend. The number of columns will correspond to the eventual number of channels in a completed recording head. A ceramic block is prepared which corresponds to the dimensions of the substrate. Channels or notches are cut into the ceramic block so that the substrate columns engage them in a male/female relationship. The channels allow for the entirety of the column to be accepted within the channel so that the base of the substrate flushly abuts the corresponding base of the ceramic block. The ceramic block is then adhered to the substrate. In particular, the columns of the substrate are glass bonded to the interior of the channels in the ceramic block, thus forming a head member.
The top and bottom of the head member are then cut or ground to produce a uniform block of alternating ceramic portions and substrate columns wherein each substrate column includes a sub-gap. A sufficient amount of the head member is cut or ground so that the substrate columns extend through the entire height of the remaining portion of the head member. During this process, the upper portion of the head member can be appropriately radiused, as it is this section which will become the tape bearing surface of the writing head.
If desired, air bleed slots can be cut into the head member, perpendicular to the direction of tape travel. On top of the head member, a magnetically permeable thin film is deposited, preferably by a sputtering process. It is within this thin film that the writing gaps will be produced. As such, any known process of forming these gaps in the thin film is acceptable. To the extent that various plating techniques will be utilized, those gaps would be formed accordingly.
At this point, a back-bar is attached to the head member. The back-bar is formed from an appropriate magnetically permeable material, such as ferrite. The back-bar is provided with a structure so that it may be wrapped with an appropriate winding to produce a coil. The back-bar can be formed in a wide variety of configurations. In the simplest form, a single winding is provided around all of the back-bars, and when driven, will engage each of the channels simultaneously. Alternatively, a separate winding is provided for each channel, thus allowing each channel to be separately driven and controlled. Furthermore, any intermediary combination is allowable. That is, any particular combination of channels can be tied together. When the channels are timed and driven independently, sections of the magnetically permeable thin film must be removed between the channels. This prevents magnetic flux from passing from one channel to another through the thin film layer. It is the prevention of this cross talk which allows the multi-channel head to have its channels driven independently in time or phase. To produce such isolation, sections of the thin film can be removed by ion milling, wet chemical etching, or by any other known process. Other techniques such as selective plating or selective spattering could also be used.
In one embodiment, the present recording head provides a magnetically impermeable barrier between each channel so that actuation of one channel will in no way interfere with any other channel in the head. Hence, a significant portion of the magnetic volume of the head laying between each channel has been replaced with a ceramic material. Furthermore, in the back-gap area a back-bar has been incorporated. Utilization of the back-bar serves to reduce the reluctance of the back-gap and increase the efficiency of the head. Due to the reduction in overall volume of the ferrite core in the interchannel area, the head has a lower total inductance and is therefore more easily driven. Due to the lower inductance per channel, the frequency response of the head can be greatly increased. This increased response time translates into faster current rise times for the output flux signal generated. This ultimately leads to sharper written transitions when the head is used to apply servo patterns to magnetic media. It also allows for specific patterns to be accurately and quickly written by individually controlling and driving the various channels of the head.
In another aspect of the present invention, the magnetically permeable thin film layer is optimally configured to complete a magnetic circuit for each channel, while limiting mechanical interference of the film with the air bleed slots. Consideration must be given to the minimal requirements for completing the circuit and the engagement of the media against a head having a non-planar surface while minimizing the complexity of providing the air bleed slots. In addition, when working with components of this scale, consideration must be given to the etching or milling technique utilized to impart and define the thin film layer so that mechanical shear or peeling of the film is not induced by the tape""s motion.
It is an object of the present invention to provide a multi-channel magnetic surface film servo write head having a reduced volume of magnetically permeable material.
It is a further object of the present invention to reduce the reluctance of the back-gap portion of the magnetic recording head.
It is another object of the present invention to provide a magnetic recording head having multiple channels wherein each channel is separately and individually controllable.
It is still another object of the present invention to provide a method of making a multi-channel head wherein each channel is isolated from the next.
It is yet another object of the invention to provide a highly efficient multi-channel recording head having a relatively high frequency response suitable for use as a servo write head.